In a conventional private branch exchange (PBX) network, the intelligence of the system generally resides within the PBX, while the phone is a simple terminal. The PBX allows the phone to make and receive simple telephone calls, and also provides numerous additional services such as, for example, dial-by-name, voice mail, speed dial, call transfer, display of caller information, and the like. Such PBX functionality is generally achieved by the exchange of digital, vendor-specific, private signaling codes between the phone and the PBX.
The underlying transport mechanism for the private signaling code exchange may be a direct point-to-point digital connection, or via an internet protocol (IP) based networked connection. In an IP based networked connection, the telephone is generally an IP telephone or a digital telephone with an IP translator, either of which is capable of IP telephony communication.
FIG. 1 is a schematic block diagram of a typical IP telephony network. The network includes a telephone 10 coupled to a PBX 12 over an IP network 14. The telephone 10 preferably communicates with the PBX 12, and accesses the full host of services provided by the PBX by transmitting private signaling codes over the IP network 14. For example, when a button on the telephone 10 is pressed, the corresponding private signaling code is transmitted from the telephone to the PBX 12 to request the PBX to perform an operation for the phone, such as, for example, a call hold operation. The telephone 10 and the PBX 12 are preferably coupled to one or more IP translators 16, 20 translating between private code/voice signaling and IP signaling. The IP translator 16 coupled to the telephone 10 receives private signaling codes transmitted by the phone, encapsulates the codes within IP packets, and transmits these IP packets over the IP network 14. The private signaling code packets are typically transport layer TCP or UDP packets. Digital voice transmitted by the telephone 10 is converted into packetized voice, such as, for example, real time protocol (RTP) packets, which are also then transmitted over the IP network 14 via the IP translator 16.
A recipient translator 20 receives the IP packets, decapsulates the private signaling codes, and forwards these codes to the PBX 12 for invoking a particular PBX function for the telephone 10. The RTP voice packets are also decompressed, converted to standard digital voice, and sent to the PBX.
Some PBX systems similarly accomplish the overall function of converting between packetized and digital data using IP translators which are internal to the PBX and telephone devices, instead of the external translators 16, 20 as shown in FIG. 1.
A major deficiency with the network illustrated in FIG. 1 is that the private signaling codes transmitted in traditional IP packets by the IP translators 16, 20 are generally vendor-specific and unidentifiable by other third-party devices. Powerful third-party SIP services such as intelligent call redirection, one-number forwarding, integration with presence and instant messaging functions, and other such services are incompatible with the private signaling code IP packets.
Another deficiency with existing networks is that other IP phones that adhere to standardized application-layer control protocols, such as H.323 and session initiation protocol (SIP), do not utilize private signaling codes for communicating with a specific PBX. Further, although H.323/SIP phones can communicate with some PBXs, the PBX services provided are generally simple, such as, for example, making or receiving a call. The H.323 and SIP phones are generally not exposed to the full suite of PBX services accessible only via private signaling codes.
Accordingly, it is desirable to have a telephone system that may take advantage of the full integrated suite of services provided by both the PBX system and the SIP network. Such an integrated system allows the PBX system to take advantage of existing and emerging third-party SIP services. It also allows SIP hardware-based IP phones and software-based user agents, to take full advantage of the PBX services.